errseq: Add to documentation tree

- Move errseq.rst into core-api
 - Add errseq to the core-api index
 - Promote the header to a more prominent header type, otherwise we get three
   entries in the table of contents.
 - Reformat the table to look nicer and be a little more proportional in
   terms of horizontal width per bit (the SF bit is still disproportionately
   large, but there's no way to fix that).
 - Include errseq kernel-doc in the errseq.rst
 - Neaten some kernel-doc markup

Signed-off-by: Matthew Wilcox <mawilcox@microsoft.com>
Reviewed-by: Jeff Layton <jlayton@redhat.com>
Reviewed-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/core-api/errseq.rst

@@ -1,5 +1,7 @@
+=====================
 The errseq_t datatype
 =====================
+
 An errseq_t is a way of recording errors in one place, and allowing any
 number of "subscribers" to tell whether it has changed since a previous
 point where it was sampled.
@@ -21,12 +23,13 @@ a flag to tell whether the value has been sampled since a new value was
 recorded.  That allows us to avoid bumping the counter if no one has
 sampled it since the last time an error was recorded.
 
-Thus we end up with a value that looks something like this::
+Thus we end up with a value that looks something like this:
 
-    bit:  31..13        12        11..0
-    +-----------------+----+----------------+
-    |     counter     | SF |      errno     |
-    +-----------------+----+----------------+
++--------------------------------------+----+------------------------+
+| 31..13                               | 12 | 11..0                  |
++--------------------------------------+----+------------------------+
+| counter                              | SF | errno                  |
++--------------------------------------+----+------------------------+
 
 The general idea is for "watchers" to sample an errseq_t value and keep
 it as a running cursor.  That value can later be used to tell whether
@@ -42,6 +45,7 @@ has ever been an error set since it was first initialized.
 
 API usage
 =========
+
 Let me tell you a story about a worker drone.  Now, he's a good worker
 overall, but the company is a little...management heavy.  He has to
 report to 77 supervisors today, and tomorrow the "big boss" is coming in
@@ -125,6 +129,7 @@ not usable by anyone else.
 
 Serializing errseq_t cursor updates
 ===================================
+
 Note that the errseq_t API does not protect the errseq_t cursor during a
 check_and_advance_operation. Only the canonical error code is handled
 atomically.  In a situation where more than one task might be using the
@@ -147,3 +152,8 @@ errseq_check_and_advance after taking the lock. e.g.::
 
 That avoids the spinlock in the common case where nothing has changed
 since the last time it was checked.
+
+Functions
+=========
+
+.. kernel-doc:: lib/errseq.c

Documentation/core-api/index.rst

@@ -22,6 +22,7 @@ Core utilities
    flexible-arrays
    librs
    genalloc
+   errseq
    printk-formats
 
 Interfaces for kernel debugging

include/linux/errseq.h

@@ -1,6 +1,6 @@
 /* SPDX-License-Identifier: GPL-2.0 */
 /*
- * See Documentation/errseq.rst and lib/errseq.c
+ * See Documentation/core-api/errseq.rst and lib/errseq.c
  */
 #ifndef _LINUX_ERRSEQ_H
 #define _LINUX_ERRSEQ_H

lib/errseq.c

@@ -46,14 +46,14 @@
  * @eseq: errseq_t field that should be set
  * @err: error to set (must be between -1 and -MAX_ERRNO)
  *
- * This function sets the error in *eseq, and increments the sequence counter
+ * This function sets the error in @eseq, and increments the sequence counter
  * if the last sequence was sampled at some point in the past.
  *
  * Any error set will always overwrite an existing error.
  *
- * We do return the latest value here, primarily for debugging purposes. The
- * return value should not be used as a previously sampled value in later calls
- * as it will not have the SEEN flag set.
+ * Return: The previous value, primarily for debugging purposes. The
+ * return value should not be used as a previously sampled value in later
+ * calls as it will not have the SEEN flag set.
  */
 errseq_t errseq_set(errseq_t *eseq, int err)
 {
@@ -108,11 +108,13 @@ errseq_t errseq_set(errseq_t *eseq, int err)
 EXPORT_SYMBOL(errseq_set);
 
 /**
- * errseq_sample - grab current errseq_t value
- * @eseq: pointer to errseq_t to be sampled
+ * errseq_sample() - Grab current errseq_t value.
+ * @eseq: Pointer to errseq_t to be sampled.
  *
  * This function allows callers to sample an errseq_t value, marking it as
  * "seen" if required.
+ *
+ * Return: The current errseq value.
  */
 errseq_t errseq_sample(errseq_t *eseq)
 {
@@ -134,15 +136,15 @@ errseq_t errseq_sample(errseq_t *eseq)
 EXPORT_SYMBOL(errseq_sample);
 
 /**
- * errseq_check - has an error occurred since a particular sample point?
- * @eseq: pointer to errseq_t value to be checked
- * @since: previously-sampled errseq_t from which to check
+ * errseq_check() - Has an error occurred since a particular sample point?
+ * @eseq: Pointer to errseq_t value to be checked.
+ * @since: Previously-sampled errseq_t from which to check.
  *
- * Grab the value that eseq points to, and see if it has changed "since"
- * the given value was sampled. The "since" value is not advanced, so there
+ * Grab the value that eseq points to, and see if it has changed @since
+ * the given value was sampled. The @since value is not advanced, so there
  * is no need to mark the value as seen.
  *
- * Returns the latest error set in the errseq_t or 0 if it hasn't changed.
+ * Return: The latest error set in the errseq_t or 0 if it hasn't changed.
  */
 int errseq_check(errseq_t *eseq, errseq_t since)
 {
@@ -155,11 +157,11 @@ int errseq_check(errseq_t *eseq, errseq_t since)
 EXPORT_SYMBOL(errseq_check);
 
 /**
- * errseq_check_and_advance - check an errseq_t and advance to current value
- * @eseq: pointer to value being checked and reported
- * @since: pointer to previously-sampled errseq_t to check against and advance
+ * errseq_check_and_advance() - Check an errseq_t and advance to current value.
+ * @eseq: Pointer to value being checked and reported.
+ * @since: Pointer to previously-sampled errseq_t to check against and advance.
  *
- * Grab the eseq value, and see whether it matches the value that "since"
+ * Grab the eseq value, and see whether it matches the value that @since
  * points to. If it does, then just return 0.
  *
  * If it doesn't, then the value has changed. Set the "seen" flag, and try to
@@ -170,6 +172,9 @@ EXPORT_SYMBOL(errseq_check);
  * value. The caller must provide that if necessary. Because of this, callers
  * may want to do a lockless errseq_check before taking the lock and calling
  * this.
+ *
+ * Return: Negative errno if one has been stored, or 0 if no new error has
+ * occurred.
  */
 int errseq_check_and_advance(errseq_t *eseq, errseq_t *since)
 {